Currently, two different medical approaches are used in order to increase the growth rate of farm animals: On the one hand the administration to farm animals of antibiotics or antibiotic-like compounds, and on the other the administration of growth hormones.
The administration of antibiotics and antibiotic-like compounds in farm animals (notably pigs) in order to promote the growth rate of these animals has in later years been proven to cause several problems. Some of these compounds are chemically closely related to antibiotics used for treatment of human disease, and evidence is building up that extensive use of such compounds in farm animals induces cross-resistance towards human antibiotics in micro-organisms pathogenic in man. Further, relatively low increases in growth rates of 1–3% are obtained with these compounds.
The use of growth hormones in farm animals is expensive, and the treatment has to be repeated at rather short intervals due to the relatively short half-life of growth hormone. Furthermore, the presence of potential residual growth hormone in meat produced from treated animals has created some concerns in European consumers particularly.
GDF-8
GDF-8 or myostatin was discovered in May 1997 as a growth regulating factor selectively down-regulating skeletal muscle growth (McPherron et al., Nature, 387, 83–90, 1997). GDF-8 expression is restricted to the myotome compartment of developing embryonic somites, but it is also expressed in various muscle tissues throughout the body in the adult animal.
GDF-8 knock-out mice exhibit a strongly increased skeletal muscle mass. The increase in skeletal muscle mass appear to be widespread throughout the body, and isolated muscles from GDF-8 negative mice weigh about 2–3 times more than wild type muscles. The total body weights of the knock-out mice are about 35% higher than wild type mice and mice lacking the GDF-8 gene has more than 80% more muscle fibres compared to normal mice. The massive skeletal muscle enlargement observed in the knock-out mice is, however, not merely due to an increase in muscle fibre numbers but also to a significant muscle fibre hypertrophy. The cross-sectional muscle area of the GDF-8 knock-out mice is increased by about 14 to 49% depending on muscle type.
Interestingly, there is also observed an enhanced rate of muscle mass increase in adult transgenic mice compared to adult non-transgenic mice. Further, all GDF-8 negative mice has been shown to be viable and fertile.
In November 1997 the authors who originally discovered GDF-8 published that two breeds of cattle that are characterized by strongly increased muscle mass, Belgian Blue and Piedmontese, have mutations in the GDF-8 coding sequence and that this accounts for their large muscles (McPherron and Se-Jin Lee, 1997, PNAS 94, 12457–12461). This phenomenon of “double muscling” has been observed in many breeds of cattle for the past 190 years, and the animals have an average increase in muscle mass of 20–25%. They also show an increased feed efficiency, but they still produce high-quality meat.
Unlike the GDF-8 knock-out mice, however, the Belgian Blue cattle also exhibit a reduction in mass of most other organs. These “natural knock-out cows” also suffer from reduced female fertility, reduced viability of offspring, and a delayed sexual maturation.
The relative increase in muscle mass in “knock-out cows” is not nearly as pronounced as is observed in the knock-out mice—in fact, it corresponds to the extent of muscle hypertrophy—observed in the mice. McPherron et al. speculate that one reason could be that normal cattle may be nearer than mice to a maximum limit of muscle size (and hence to the maximum obtainable number of muscle fibres) after generations of selective breeding. No data regarding the number of muscle fibre hyperplasia versus hypertrophy in cattle was published by the authors, but based on this assumption and the muscle hypertrophy observed in the knock-out mice, it could be possible that the increase in muscle mass and growth rate observed in e.g. the Belgian Blue cattle is largely due to muscle hypertrophy and to a lesser extent muscle hyperplasia.
Physiological Role of GDF-8
Expression of GDF-8 is highly restricted to skeletal muscle. There is a low level of expression in adipose tissue, but notably there is no expression in heart muscle. The physiological role of GDF-8 in the adult individual is not known, although it seems that GDF-8 may function as a specific negative regulator of skeletal muscle growth. Speculations about the physiological role centres upon important functions in exercise induced muscle hypertrophy or regeneration after muscle injury. GDF-8 may, however, also suppress adipose tissue growth. It is not known whether GDF-8 works locally or systemically during the growth of the animal.
Structure of GDF-8
GDF-8 belongs to the transforming growth factor β (TGF-β) super family which encompasses a group of structurally-related proteins involved in embryonic development. Human and bovine GDF-8 are produced as 375 amino acids long precursor proteins. As other TGF-β super family proteins GDF-8 is probably processed proteolytically into a much shorter C-terminal fragment of about 109 amino acids which form disulphide linked homodimers. The homodimer is probably the biologically active form of GDF-8.
The amino acid sequences of murine, rat, human, baboon, bovine, porcine, sheep, chicken and turkey GDF-8 are known and the GDF-8 molecule is highly conserved (McPherron and Se-Jin Lee, PNAS, 94, 12457–12461, 1997). The sequences of murine, rat, human, porcine, chicken and turkey GDF-8 are 100% identical in the C-terminal region, which probably contains the biologically active part of GDF-8. Both bovine and sheep GDF-8 only differ by two amino acid residues from human GDF-8 in the C-terminal region. Bovine GDF-8 has the sequence —Glu—Gly— instead of —Lys—Glu— in positions 356–357, and sheep GDF-8 has two conservative substitutions (a Val in position 316 and an Arg in position 333 instead of Leu and Lys, respectively). None of the known GDF-8 proteins include potential N-glycosylation sites in their active C-terminal region.